you can hold for as many levels as you want. but I think 10 was the max the team would allow, but things change over time

How long can I hold a lvl?

for as long as you like, but keep in mind that some systems will take the banked xp away, and they will be lost or if you find a kind DM then they might be willing to restore it. Think that one of these system that is taking banked xp away was crafting, not sure mind you.

Will the xp acquired disappear after I take my next lvl? or am I going to end up gaining multiple lvls at once?